Remote sensing coherent ladar (laser radar) imaging systems can provide images at ranges far beyond the useful range of a traditional diffraction limited imaging systems. Coherent ladar relies on phase sensitive measurements to resolve an object and provide information about its motion. To overcome the diffraction limitations of a finite aperture system, synthetic aperture ladar (SAL) systems use multiple spatial samples from a single small aperture in motion to synthesize a larger effective aperture. This results in higher spatial resolution, but only in the along-track dimension (i.e., in the direction of flight) in which the samples were collected. Conventional SAL imaging has been demonstrated both in the laboratory as well as in flight, but is sensitive to undesired platform motions.
Apertures separated in the cross-track dimension (i.e., the direction perpendicular to both the range and direction of flight) can be used to implement support interferometric SAL (IFSAL). IFSAL can provide enhanced cross-track resolution and can be implemented in the along-track dimension to provide differential SAL (DSAL) that is substantially insensitive to platform motion.
Interferometric synthetic aperture radar (IFSAR) is a frequently used technique developed to realize enhanced resolution in the cross-track, or vertical, dimension. IFSAR utilizes the phase between each resolution cell of two synthetic aperture images collected from two closely spaced apertures to make an estimate of the cross-track position, or height, of an object. Implementing IFSAR techniques at optical wavelengths is extremely challenging and can be impacted by phase variations from a variety of sources, including variations between the two apertures, target and atmospheric de-correlation, and target motion.
Synthetic aperture ladar is extremely sensitive to uncompensated motion due to the short wavelength of the transmit laser. Along-track Differential SAL (DSAL) has been proposed as a way to produce SAL images that are immune to both platform translation and vibration. A CDMA multi-aperture ladar can implement DSAL using a single transmit and receive channel and slightly modified image formation processing. In DSAL, two apertures are separated in the along-track direction.
Range-Doppler and Micro-Doppler imaging detects and characterizes the macro-translational velocity and micro-velocity of a target. Use of multiple transceiver apertures can help to isolate and characterize these motions.